1. Field of the Invention
The present invention relates to a hydraulic circuit for a working vehicle having a plurality of actuators and a plurality of hydraulic pumps.
2. Description of the Prior Art
A tractor, for example, has, besides a hydraulic device (main actuator) for raising and lowering a working implement attached rearwardly, a hydraulic device (auxiliary actuator) for actuating a cane cutter or a grass cutter and a loader implement. The respective actuators receive pressure oil from two hydraulic pumps driven by an engine.
The auxiliary actuator is interchangeable. When the loader implement is used, for example, it is possible to shorten a cycle time by increasing oil supply. A greater quantity of oil may be required in this case than when the cane cutter or the like is used. However, this requirement is not met since the pump for driving the auxiliary actuator provides a fixed displacement.
It is relatively easy to supply the oil discharged from the main actuator confluently with the oil from the auxiliary actuator. However, oil lines extending to the respective actuators include relief valves. Where the two relief valves have the same relief pressure, the confluence of the oils from the two pumps may cause resonance of the relief valves and produce chattering sounds. Thus, the oils cannot simply be joined to meet the above requirement.
Various hydraulic devices have approximately the same relief pressure and to increase the relief pressure for certain hydraulic devices impairs flexibility. On the other hand, the relief pressure may be freely selected for the hydraulic implements of the tractor to which the above hydraulic devices are applied. It is known that a high system pressure enables compact designing.